ABSTRACT-The hemagglutinating activity (HA) of Edwardsiella tarda, which had been isolated from cultured fish and culture environments, was investigated in relation to NaCl concentration of the growth medium. E. tarda cells were cultured in a peptone-yeast extract broth supplemented with 3% NaCl (3%-NaCl culture) and without NaCl (0%-NaCl culture). Hemagglutination assays with guinea pig erythrocytes classified the strains into three HA patterns. Seventeen strains exhibited HA only with the 3%-NaCl culture (type A). A more frequent type (35 strains) displayed HA in both 0%-and 3%-NaCl cultures but the 3%-NaCl culture showed higher HA activity than the 0%-NaCl culture (type B). No HA was detected in both cultures of the other three strains (type C). The NaCl-induced HA was well correlated with the expression of a 19.3 kDa protein, a fimbrial major subunit (FimA). Infection experiments with a selected strain (type A) of E. tarda revealed that the 3%-NaCl culture was more virulent to Japanese flounder Paralichthys olivaceus than the 0%-NaCl culture, when fish were challenged by an oral route. This induction of the fimbrial protein under higher NaCl concentration may play an important role in the virulence of E. tarda in marine environments.